Update: No reply yet from the PPA maintainer (PandaJim or from the Audacity Team) but forum user thx-1138 has a work-around posted:
https://forum.audacityteam.org/t/can-no-longer-enable-plugins-after-2-3-3-2-4-0-update/57039/13
it worked for me on a laptop using Mint 18.04, I haven’t yet tried that on my Mint 19.04 as I am in the middle of a project. But your mileage may vary, so proceed accordingly.